1. Field of the Invention
The present invention relates to an erasing device and the like of an image pattern recorded on a display medium. More specifically, the present invention relates to an erasing device and the like for erasing an image pattern recorded on an optical writing type display medium that uses a cholesteric liquid crystal layer and a photoconductive layer.
2. Description of the Related Art
Among rewritable display media of super low power consumption capable of keeping displays with no power supply, the display medium using the cholesteric liquid crystal can display color images without using color filters. Therefore, developments of such display medium have been actively conducted.
The cholesteric liquid crystal employs a layer structure in which molecules are arranged in layers of a pseudo manner, which is a spiral structure where the major axes of liquid crystal molecules are arranged in parallel in each layer, and the alignment directions thereof are rotated by each layer. The cholesteric liquid crystal has a selective reflection characteristic which reflects the wavelength corresponding to the spiral pitch among light making incident in parallel to the spiral axis and transmits the remaining light. The wavelength of the reflection light can be controlled through adjusting the spiral pitch. Thus, it is possible to achieve a display medium capable of providing a full-color display by laminating cholesteric liquid crystals, each of which selectively reflects the light of three colors such as red, green, and blue, for example.
The cholesteric liquid crystal has three kinds of alignments, i.e., the planar alignment, the focal conic alignment, and the homeotropic alignment. With the planar alignment, the spiral axis is almost in parallel with respect to the incident light, and reflects the light that corresponds to the spiral pitch. With the focal conic alignment, the spiral axis is almost perpendicular with respect to the incident light, and the selective reflection characteristic is lost so that incident light is transmitted therethrough. With the homeotropic alignment, the spiral structure is broken, so that the selective reflection characteristic is lost. Therefore, the incident light is transmitted therethrough. Those three kinds of alignments are switched by applying voltages to the cholesteric liquid crystals. The planar alignment and the focal conic alignment keep the states thereof without applying voltages. Thus, image patterns can be kept without a power supply through settling a region of the planar alignment and a region of the focal conic alignment according to the image pattern.
As a method for writing the image pattern on the display medium using the cholesteric liquid crystal, an optical writing type driving device is disclosed (Japanese Unexamined Patent Publication Hei 11-237644 (Patent Document 1), for example). This display medium is in a structure in which a cholesteric liquid crystal layer and a photoconductive layer are laminated, and those are sandwiched by a pair of electrodes. This display medium is loaded on the writing device, and light according to the image pattern is irradiated from an exposure device within the writing device. At the same time, a signal is inputted to the display medium from the writing device, and a voltage is applied between the electrodes by that signal. The resistance of the photoconductive layer in the region where the light is irradiated becomes reduced, so that a divided voltage applied to the cholesteric liquid crystal layer becomes increased. Inversely, there is no change in the resistance of the photoconductive layer in the region where the light is not irradiated, so that the divided voltage applied to the cholesteric liquid crystal layer becomes relatively small. As described, there is generated a difference in the voltage applied to the cholesteric liquid crystal layer in accordance with the brightness of the light, and the alignment in the cholesteric liquid crystal layer changes thereby. This makes it possible to write the image pattern. Even if the display medium is detached from the writing device, the image pattern can be held. Thus, it is possible to take out only the display medium and view the image pattern. That is, as in cases of paper media, it is possible to achieve a portable and rewritable display medium.
There is considered a case where an image pattern is written to an optical writing type display medium, which is taken out and the image thereon is viewed, and the image pattern needs to be erased thereafter for preventing information leakage. However, the display medium itself does not have a writing function. Thus, when there is no writing device, it is not possible to overwrite and erase the image pattern. A surface emission type display such as an LCD (Liquid Crystal Display) that creates optical image patterns are most frequently used for the exposure device, and to carry the writing device including the exposure device in case for erasing the image patterns results in losing the portability of the display medium that is an original characteristic of the display medium.
As an image pattern erasing module that does not require a writing device including an exposure device, there is disclosed a display medium that includes a clip having an erasing function (Japanese Unexamined Patent Publication 2006-039107 (Patent Document 2)). This clip functions as an erasing device of the display medium. That is, in response to an action of pulling out the clip from the display medium, an erasing voltage is applied from an erasing power supply provided to the clip, and an image pattern recorded on the display medium is erased. Only thing required for erasure is the clip, so that the portability of the display medium can be maintained.
Further, Japanese Unexamined Patent Publication 2001-201762 (Patent Document 3) discloses an optical writing type display medium having a specific display function layer. This display function layer is a structure in which a laminate including a photoconductive layer and a cholesteric liquid crystal layer constituted with a plurality of layers is sandwiched between a pair of electrodes.
Japanese Unexamined Patent Publication 2003-186052 (Patent Document 4) discloses an electronic display medium on which image information is recorded by impression a voltage and irradiation of light (see paragraph 0009). This electronic display medium is formed by laminating a liquid crystal layer constituted with a cholesteric liquid crystal and the like and a photoconductive layer whose resistance value is changed by absorption of light (see paragraph 0017). Further, voltage value Va applied to the liquid crystal layer can be expressed by a following expression, provided that a static capacitance value of the liquid crystal layer is Ca, a static capacitance value of the photoconductive layer is Cb, a resistance value of the liquid crystal layer is Ra, the resistance value of the photoconductive layer is Rb, and a voltage value applied to the liquid crystal layer and the photoconductive layer is V (see paragraphs 0019, 0020).Va=V×Ca/C=V/(1+Rb/Ra), whereC=CaCb/(Ca+Cb)
Japanese Unexamined Patent Publication 2006-039833 (Patent Document 5) discloses an optical writing type electronic paper (see paragraph 0032). In this electronic paper, a clip is attached to a display unit in a detachable manner. The clip includes a terminal connected to electrodes of the display unit, a radio communication unit, a memory, a processor, a battery, and the like, and functions as a processing unit.
Japanese Unexamined Patent Publication Hei 11-326871 (Patent Document 6) discloses a liquid crystal display element driving method which displays images by sandwiching a liquid crystal showing a cholesteric phase between two substrates having electrodes provided in matrix on the surfaces thereof, and changing the state of the liquid crystal with a voltage applied to the electrodes.
As in the case of the paper medium, the optical writing type display medium is used in various kinds of places. Thus, erasing operations are required to be performed regardless of the places. Therefore, the power supply provided to the clip as the erasing device disclosed in Patent Document 2 needs to be a battery such as a dry battery. In order for the erasing device to have a similar portability as that of the optical writing type display medium, the volume of the battery provided to the erasing device is preferable to be small. Further, the number of exchanging times of the battery is preferable to be small, so that it is unnecessary to carry a spare battery.
That is, the power consumed in one-time erasure is preferable to be as small as possible. However, Patent Documents 1-6 mention about nothing about that, while the power consumption can be suppressed if it is possible to set the erasing voltage applied to the display medium by the erasing device to be low.
It is therefore an exemplary object of the present invention to provide an erasing device in which the power consumption required for the erasing operation is suppressed, and with which the portability of the optical writing type display medium is not ruined.